1. Field of the Invention
This invention relates to a process and apparatus for removing a metal carbonyl composition and moisture from a gas. More particularly, this invention relates to a process and apparatus for removing from a gas a metal carbonyl composition and moisture present in a gas to be treated, as well as moisture formed during removal of metal carbonyl composition from the gas.
2. Description of Prior Art
At the present time, numerous industries require the use of purified gases in their processes. Even parts per million (ppm) or parts per billion (ppb) levels of contaminants can have deleterious effects on the appearance or performance of certain products. The 1994 Semiconductor Industry Association (SIA) recommendation for semiconductor processing established key impurity level specifications in bulk and specialty gases for 0.25 .mu.m processing of 0.1-1.0 and 1-100 ppb, respectively. The individual metal on silicon wafer specification for the same device geometry was set at 2.5.times.10.sup.10 atoms/cm.sup.2. Because of these requirements and other observed adverse effects of using unpurified gases, the purification of process and purge gases in the microelectronics industry has found widespread acceptance and usage. The ideal purification material would remove the unwanted impurities without generating volatile by-products that would contaminate the gas. Also, the purification material should not be volatile under vacuum or pressurized process flow conditions. It would additionally be desirable for the purification material to chemically convert any toxic impurities into less toxic, nonvolatile by-products. Ideally, the purification material would operate at room temperature and not require electrical power for operation.
Lindley, et al Solid State Technol. 1997, 40 (8), 93 describes the enhanced selectivity for dielectric etch when CO is incorporated in the etch chemistry. This selectivity is required for production of semiconductor devices with geometries .ltoreq.0.35 .mu.m. Unfortunately, the reaction of CO with certain metals, e.g., iron and nickel, is thermodynamically favored to form volatile metal carbonyls that can deposit on the wafer resulting in undesired metal contamination. This was observed in Cooper, G. Semiconductor Intl. 1997, 20 (8), 301 that describes experimental results showing increased levels of metals, e.g., iron, chromium and nickel, contamination on silicon wafers when CO is packaged in steel vs. aluminum gas cylinders. However, there was still observed an increase in nickel contamination using the aluminum cylinder versus the unetched control silicon wafers that was thought to be due to reaction of CO with the nickel in the Hastelloy tubing and nickel gaskets used in the study. Xu, et al J. Mol. Catal. 1993, 83, 391 describe the ready formation of iron pentacarbonyl, Fe(CO).sub.5, from the reaction of CO and the stainless steel surface of a FTIR cell. In addition, both Fe(CO).sub.5 and nickel tetracarbonyl (Ni(CO).sub.4) are very toxic with Threshold Limit values (TLV) of 100 and 50 ppb, respectively.
Golden, et al Ind. Eng. Chem. Res. 1991, 30 (3), 502 describe the adsorption of trace levels of Fe(CO).sub.5 and Ni(CO).sub.4 using five commercially available adsorbents, i.e., activated carbon, silica gel, activated alumina, a methanol catalyst and HY zeolite. The activated carbon and zeolite showed the largest adsorption capacities for the carbonyls and were further investigated for carbonyl desorption at elevated temperatures. The desorption of Fe(CO).sub.5 was observed from the zeolite and only a slight amount (&lt;0.1%) from the carbon; however, Ni(CO).sub.4 was completely desorbed from the carbon at elevated temperatures. In addition, the presence of other impurities, e.g., carbon dioxide (CO.sub.2), in the gas was shown to reduce the adsorption capacity of the adsorbents towards the metal carbonyls. The removal of these metal carbonyls via physisorptive processes with the concomitant concentration of these toxic impurities within a purifier will result in a safety and health hazard when the time comes to replace the purifier.
U.S. Pat. No. 4,608,239 describes a process for removing Fe(CO).sub.5 from CO containing gases using a scavenger consisting of an alkali metal hydroxide in conjunction with a high boiling hydroxylic solvent. The use of solvents is undesirable because the vapors from these solvents will contaminate the downstream products with carbon and oxygen impurities under vacuum or pressurized flow conditions.
International Patent Application Publication No. WO 94/25142 describes a scavenger of lead oxide, PbO, dispersed upon a support to remove metal carbonyls from a gas. In this application there is some uncertainty as to whether the carbonyls are removed by chemical or physical means. However, from the data, it may be simply by physisorption since black deposits were only observed when the temperatures were &gt;100.degree. C., i.e., decomposition of the carbonyl. Irreversible removal of the toxic carbonyls by chemisorption is desired from a safety standpoint. While PbO has sufficient reduction potential to oxidize iron and nickel carbonyls to their respective salts, the PbO will concomitantly be reduced to lead metal with the generation of moisture impurity as shown below in the case of Fe(CO).sub.5 : EQU Fe(CO).sub.5 +2PbO+2H.sup.+ =FeO +2Pb+H.sub.2 O+5CO
The hydrogen ion is commonly found present in support materials and gases as a contaminant. The release of moisture impurity into the purified gas stream is not desirable since moisture can have deleterious effects in certain manufacturing processes, e.g., semiconductor manufacturing.
Czech. Patent No. CS 187947 B describes a scavenger for Fe(CO).sub.5 and sulfur compounds from gas mixtures consisting of a catalyst having as its principal components, Cu and/or CuO and ZnO. These scavengers have the same undesirable characteristics, i.e., release of moisture impurity, as do the scavengers described in International Patent Application No. WO 94/25142.
U.S. Pat. No. 4,853,148 describes scavengers for removing water impurity consisting in one case of a metal halide compound dispersed in a support having formula MXY, where M is a y-valent metal selected from the group consisting of lithium (I), beryllium (II), magnesium (II), calcium (II), strontium (II), barium (II), cadmium (II), nickel (I), iron (II), iron (III), zinc (II), and aluminum (III); and y is an integer whose value is from one to three. This scavenger has the advantage over other adsorbents, e.g., zeolites and alumina, in that there is no competition for adsorption of moisture with the gas to be purified or other gas impurities, e.g., CO.sub.2. The scavenger disclosed by this patent is only capable of removing moisture and not other impurities, e.g., metal carbonyls and sulfur compounds.
It would be desirable to provide a composition suitable for removal of volatile metal carbonyls either alone or together with other impurities such as sulfur compounds from a gas while avoiding release of moisture into the gas. It would additionally be desirable to reduce the moisture concentration of the gas. It would also be desirable that such removal is accomplished by chemisorption and not physisorption mechanisms to prevent reversible release of the sorbed composition. Furthermore, it would be desirable to provide such a composition which is effective for use at normal operating pressure and room temperature.